The code originally intended to achieve this by simply waiting on flushing_cs,
and having the application thread hold flushing_cs while seeking. Unfortunately,
this can result in starvation of the application thread, since the streaming
thread always reacquires flushing_cs immediately after releasing it. Avoid this
by using a separate condition variable.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=53403Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>